In contemporary voice and data communications systems, modulation schemes are generally selected to be most consistent with the characteristics of the particular transmission channel. As a result, a modulation type used for a wireless high frequency (HF) channel, for example, may not be the one best suited for a wire-line channel. In addition, the signaling, such as the information exchange that takes place during call set-up, is often matched to the characteristics of the channel type or application. For example, the start of signaling for HF transmissions usually contains Doppler tones that allow the determination of Doppler shift. These Doppler tones are used to correct for Doppler shifts in the transmitting frequency due to the velocity of the moving vehicle containing the communications equipment. On the other hand, the start of signaling for wire-line transmission typically contains tones that allow for the estimation and subsequent cancellation of echoes.
Many modern communications systems attempt to provide global connectivity, in which case the transmission channel may span several media. For instance, one part of the transmission may take place over a wireless HF channel and another part over a wire-line one. Since the modulation and signalling protocols on the wireless and wire-line sides differ, a modem converter is necessary to convert modulation and signaling to the one that is appropriate for each portion of the transmission channel.
In one type of wireless to wire-line communications link, one end of the link may be in a mobile vehicle several hundred miles from an HF base station that is connected to a public telephone line. The other end of the link is a public telephone subscriber at another location. Upon the start of a call from the mobile vehicle, modulated signaling and message data is transmitted to the HF base station. Two separate modems are located at the base station, a HF modem for interfacing with the HF part of the link, and a wire-line modem connected to the public telephone line. The HF modem demodulates the data received from the mobile vehicle and modulates data to be transmitted to the mobile vehicle in the reverse direction. The modem conversion occurs as the HF modem transfers the demodulated data to the wire-line modem and vice-versa in the reverse direction. Thus each modem acts as a bit source for the other. The wire line modem then modulates the data in the wire-line format. This modulated data is then conveyed via the public telephone company to the intended subscriber at the other end of the link. In the opposite communications direction, the sequence is reversed.
In the prior art system just described, the HF modem at the base station generally receives an entire block of signalling and message data in the HF format, prior to the wire-line modem transmitting any corresponding signalling or message data in the wire-line protocol towards the wire-line subscriber. Each modem is therefore unaware of the signaling that is occurring at the other modem, and only views the other as a source or sink of data. The modems do not notify each other of ongoing signaling stages, but instead wait until appropriate data blocks are collected which are then passed on. This approach can introduce significant delays in the call set-up portion of the communication. These delays are not only annoying, particularly for voice communication, but can also cause the data sources at the ends of the link to time out.
It is therefore an object of the present invention to provide a modulation and signalling conversion system to allow voice or data communication between differing transmission channel types, which system introduces only minimal delays in the signalling and message data transfers.